Python if（）：vs if：

Question

On Code Academy there is this course where in the example they show

def speak(message):
    return message

if happy():
    speak("I'm happy!")
elif sad():
    speak("I'm sad.")
else:
    speak("I don't know what I'm feeling.")

The above example will NOT be related to the rest of the code I show. That was just an example for the if statement. Now I was under the impression that when ever writing an if statement it had to end in an (): like the above example.

However when doing the assignments this does not work:

def shut_down(s):
    if s == "yes"():
        return "Shutting down"
    elif s == "no"():
        return "Shutdown aborted"
    else:
        return "Sorry"

However this works:

def shut_down(s):
    if s == "yes":
        return "Shutting down"
    elif s == "no":
        return "Shutdown aborted"
    else:
        return "Sorry"

My question is how come the () is not needed next to the "yes" and "no " but : is still needed. I thought whenever writing an if statement it will automatically have to end with (): . In that very first example, that's how it is shown. Do you understand my confusion.

Answer 1

In the example given, happy() and sad() are functions, and as such require parentheses. The if itself does not need parentheses at the end (and it shouldn't have them)

Answer 2

No, if has nothing to do with ()

happy is a function. happy() is a call to that function. So, if happy(): tests if the happy function returns true when called.

In other words, if happy(): speak("I'm happy!") is equivalent to

result_of_happy = happy()
if result_of_happy:
    speak("I'm happy!")

Answer 3

As has been mentioned happy() / sad() are functions so they require () . In example two of your question you are comparing your value to the string "yes" because it is a string it does not require () .

Within an if statement you can use parentheses to make the code more readable and ensure certain operations are evaluated before others.

if (1+1)*2 == 4:
    print 'here'
else:
    print 'there'

Differs from:

if 1+1*2 == 4:
    print 'here'
else:
    print 'there'

Answer 4

Because string objects are not callable so what are you expecting then:

Then use lambda not that efficient tho:

def shut_down(s):
    if (lambda: s == "yes")():
        return "Shutting down"
    elif (lambda: s == "no")():
        return "Shutdown aborted"
    else:
        return "Sorry"